Steel pipe for use in reinforcement of automobile and method for production thereof

ABSTRACT

A steel tube having a composition which contains: 0.05 to 0.30% of C; 1.8 to 4.0% of Mn; Si; and Al is subjected to a diameter-reducing rolling process in which the total diameter-reduction rate is no less than 20% and the temperature at which the diameter-reducing rolling process is finished is no higher than 800 ° C., whereby a structure constituted of martensite and/or bainite or further of ferrite is obtained as a transformation product from the deformed γ. As a result, a steel tube having tensile strength of 1000 MPa or more and excellent three-point-bending property can be obtained. The composition of the steel tube of the present invention may further include at least one type of element selected from the group consisting of Cu, Ni, Cr and Mo, or at least one type of element selected from the group consisting of Nb, V, Ti and B, or at least of one type selected from the group consisting of REM and Ca.

TECHNICAL FIELD

[0001] The present invention relates to a steel tube for reinforcing aautomobile door. Specifically, the present invention relates to a steeltube which has high tensile strength and excellent three-point-bendingproperty and has, in particular, a large amount of buckling limitdeformation. The present invention also relates to a method of producingthe aforementioned steel tube for reinforcing a automobile door.

[0002] In the present invention, the “excellent three-point-bendingproperty” indicates that, in what is called “a three point bending test”in which a steel tube is placed over a pair of support tools distancedby a predetermined span L and the center portion of the steel tube ispressed by a bending tool having a curvature of radius R as shown inFIG. 1, the maximum pressing amount which buckling occurs (which will bereferred to as “the buckling limit pressing amount” hereinafter) isrelatively large, and also, in the graph representing the relationshipbetween the pressing load and the pressing amount of the steel tube(refer to FIG. 2), the area defined by “the pressing load-pressingamount curve from the start of pressing to the buckling limit pressingamount” and the amount of deformation axis (the hatched portion of FIG.2), i.e., the amount of energy absorbed by the steel tube before thebuckling occurs is relatively large. More specifically, when a steeltube of 31.8 mm φ (steel thickness being 1.6 mm) absorbs energy of 450 Jor more before the deformation reaches the buckling limit pressingamount (i.e., the buckling limit deformation amount) at a three pointbending test with the span L being 980 mm, the steel tube is regarded asa steel tube which is “excellent in the three-point-bending property”.

BACKGROUND ART

[0003] In order to ensure safety of passengers in a automobile at thetime of collision, improvement of the collision safety property of aautomobile body is increasingly on demand in recent years. Due to this,in the automobile body, increasing of the strength of the side portionof a automobile i.e., increasing of the strength of a automobile door isparticularly required and thus a bar for reinforcing a automobile dooris always provided in a automobile door, in recent years. Here, in orderto reduce the weight of a automobile body, a steel tube is increasinglyin use for the bar for reinforcing a door.

[0004] A steel tube for a automobile door reinforcing bar is required tohave high strength, so that the automobile door reinforcing bar canachieve the intended effect in application thereof. Therefore, a steeltube whose strength has been increased is generally used for aautomobile door reinforcing bar. Conventionally, a electric resistancewelded tube is used as a steel tube for automobile door reinforcing bar.Specifically, the off-line QT (quench and temper) type steel tube whosestrength has been increased by the off-line QT treatment such asinduction quenching has conventionally been used, or the as rolled typesteel tube which is produced by electric resistance welding a steelsheet having high strength has conventionally been used (here, the steelsheet is strengthed by the QT treatment at the stage of producing a thinsteel sheet as the base material of a electric resistance welded tube).

SUMMARY OF THE INVENTION

[0005] However, in the case of the off-line quench and temper (QT) typesteel tube, there is a problem that the production steps arecomplicated, a relatively long period is required for production and theproduction cost is relatively high, because the quench and tempertreatment has to be carried out at “off-line”. On the other hand, in thecase of the as rolled type steel tube, there is a problem that coldforming strain generated during tube forming tends to remain, wherebythe steel tube buckles at a relatively early stage of the three pointbending test and thus exhibits poor three-point-bending property. Inaddition, in the case of the as rolled type steel tube, since the steelsheet is subjected to the QT treatment at the stage of the thin steelsheet production and thereafter the steel tube is produced from thesteel sheet, there is a problem that the welded portion by electricresistance welding at which the ends of the steel sheet are electricresistance-welded (the induction welded portion) tends to be softeneddue to heat affection. Further, since the thin steel sheet as the basematerial of steel tube has extremely high strength, there arises aproblem that the steel tube tends to suffer from a relatively largeamount of springback at the time of tube forming, the steel tube is hardto form and the production facility must be large-scale, whereby thefacility cost becomes high.

[0006] The present invention has an object to solve the aforementionedproblems of the prior art, to propose a steel tube for reinforcing aautomobile door which has high strength (the tensile strength of nosmaller than 1000 MPa) and excellent three-point-bending property, andto propose a method of producing the same steel tube.

[0007] In order to solve the aforementioned problems, the inventors ofthe present invention have assiduously studied for means to enhancestrength and three-point-bending property of a steel tube at the sametime, without carrying out any off-line heat treatment. As a result, theinventors have found the following items. First, by subjecting a steeltube having a uniquely restricted composition to a diameter-reducingrolling process whose total diameter-reduction rate is no less than 20%,at a temperature within the “α+γ” two-phase region or slightly above theregion, and then cooling the steel tube, the structure of the steel tubebecomes a structure which includes hard martensite and bainite as maincomponents, obtained as a result of transformation of the deformedaustenite, and ferrite, in a mixed manner. By utilizing the steel tubehaving the aforementioned structure, a steel tube in which high strengthand excellent three-point-bending property are compatible withoutcarrying out the conventional, specific off-line heat treatment (quenchand temper treatment). Such significant improvement of thethree-point-bending property is achieved presumably because thestructure of the steel tube is mainly constituted of martensite orbainite which has been transformed from the deformed γ. On the otherhand, the structure of the conventional off-line QT type steel tube ismainly constituted of martensite or bainite which has been transformedfrom the reheated austenite (γ). The three-point-bending property of theconventional as rolled type steel tube, and the three-point-bendingproperty of the steel tube having a structure mainly composed ofmartensite or bainite which has been transformed from the deformed γ(the steel tube of the present invention) are shown in FIG. 3, in amanner of comparing the former with the latter. From FIG. 3, it isunderstood that the buckling limit pressing amount (the buckling limitdeformation amount) of the steel tube of the present invention isrelatively large and thus absorbs a relatively large amount of energy ascompared with the conventional steel tube.

[0008] The present invention has been achieved by further studying theaforementioned discoveries. The present invention is constituted of anovel technique whose idea is essentially different from that of theconventional steel tube for reinforcing a automobile door.

[0009] Specifically, the first aspect of the present invention providesa steel tube for reinforcing a automobile door, having a compositioncomprising: 0.05 to 0.30 mass % of C; 0.01 to 2.0 mass % of Si; 1.8 to4.0 mass % of Mn; 0.005 to 0.10 mass % of Al; and the remainder as Feand unavoidable impurities, wherein the steel tube has tensile strengthof no less than 1000 MPa and excellent three-point-bending property.Further, in the first aspect of the present Invention, it is preferablethat the steel tube has a structure which is constituted of martensiteand/or bainite or a structure which is a mixture of martensite and/orbainite and ferrite, and the martensite and/or bainite is atransformation product obtained as a result of transformation of thedeformed austenite. Yet further, in the first aspect of the presentinvention, it is preferable that the content of ferrite in thestructure, expressed as the area ratio, is no more than 20%. Yetfurther, in the first aspect of the present invention, it is preferablethat the yield ratio of the steel tube is no larger than 80%.

[0010] Yet further, in the first aspect of the present invention, it ispreferable that the steel tube has at least one composition selectedfrom the group consisting of composition A, composition B andcomposition C described below, in addition to the aforementionedcomposition.

[0011] Composition A: at least one type of element selected from thegroup consisting of: no more than 1 mass % of Cu; no more than 1 mass %of Ni; no more than 2 mass % of Cr; and no more than 1 mass % of Mo.

[0012] Composition B: at least one type of element selected from thegroup consisting of: no more than 0.1 mass % of Nb; no more than 0.5mass % of V; no more than 0.2 mass % of Ti; and no more than 0.003 mass% of B.

[0013] Composition C: at least one selected from the group consistingof: no more than 0.02 mass % of REM; and no more than 0.01 mass % of Ca.

[0014] The second aspect of the present invention provides a method ofproducing a steel tube for reinforcing a automobile door, comprising thesteps of: preparing a mother steel tube having a composition whichincludes: 0.05 to 0.30 mass % of C; 0.01 to 2.0 mass % of Si; 1.8 to 4.0mass % of Mn; 0.005 to 0.10 mass % of Al; and the remainder as Fe andunavoidable impurities; subjecting the mother steel tube to a heating orsoaking treatment; and thereafter, subjecting the mother steel tube to adiameter-reducing rolling process in which the total diameter-reductionrate is no less than 20% and the temperature at which thediameter-reducing rolling process is finished is no higher than 800° C.Further, in the second aspect of the present invention, it is preferablethat the steel tube has at least one composition selected from the groupconsisting of composition A, composition B and composition C describedbelow, in addition to the aforementioned composition.

[0015] Composition A: at least one type of element selected from thegroup consisting of: no more than 1 mass % of Cu; no more than 1 mass %of Ni; no more than 2 mass % of Cr; and no more than 1 mass % of Mo.

[0016] Composition B: at least one type of element selected from thegroup consisting of: no more than 0.1 mass % of Nb; no more than 0.5mass % of V; no more than 0.2 mass % of Ti; and no more than 0.003 mass% of B.

[0017] Composition C: at least one selected from the group consistingof: no more than 0.02 mass % of REM; and no more than 0.01 mass % of Ca.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is an explanatory diagram which shows the scheme of a threepoint bending test.

[0019]FIG. 2 is an explanatory diagram which shows the definition of thethree-point-bending absorption energy value.

[0020]FIG. 3 is a graph which shows the result of the three pointbending test of a steel tube of the present invention and the result ofthe three point bending test of a conventional steel tube.

THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION

[0021] The steel tube for reinforcing a automobile door of the presentinvention is a steel tube which has tensile strength TS of no smallerthan 1000 MPa and has excellent three-point-bending property. Inaddition, the steel tube for reinforcing a automobile door of thepresent invention preferably exhibits the yield ratio of no higher than80%. The steel tube of the present invention may be any of a weldedsteel tube such as butt-welded steel tube and electric resistance weldedtube, and seamless steel tube, and is not restricted by the method ofproducing each mother steel tube.

[0022] Next, the reason for restricting the composition of the steeltube for reinforcing a automobile door of the present invention will bedescribed. It should be note that “mass %” will be simply referred to as“%” hereinafter.

[0023] C: 0.05% to 0.30%

[0024] C is an element which is solid-solved in the base material orprecipitated as a carbide, thereby increasing the strength of steel. Inthe present invention, the content of C must be no less than 0.05%, sothat the desired strength of the steel can be reliably obtained. Whenthe content of C exceeds 0.30%, the weldability property of the steel isdeteriorated. Accordingly, in the present invention, the content of C isrestricted within the range of 0.05 to 0.30%.

[0025] Si: 0.01% to 2.0%

[0026] Si is an element which serves as a deoxidizing agent and issolid-solved in the base material, thereby increasing the strength ofthe steel. Such the effect of Si is observed when the content of Si isno less than 0.01%, preferably no less than 0.1%. However, when thecontent of Si exceeds 2.0%, the ductility of the steel is deteriorated.Accordingly, in the present invention, the content of Si is restrictedwithin the range of 0.01 to 2.0%. In order to achieve excellent balancebetween strength and ductility, the content of Si is preferably withinthe range of 0.10 to 1.5%.

[0027] Mn: 1.8% to 4.0%

[0028] Mn is an element which serves for increasing the strength of thesteel, improving the hardenability property and accelerating formationof martensite and bainite during cooling after the rolling process. Suchthe effect of Mn is observed when the content of Mn is no less than1.8%. However, when the content of Mn exceeds 4.0%, ductility of thesteel is deteriorated. Accordingly, in the present invention, thecontent of Mn is restricted within the range of 1.8 to 4.0%. In order toreliably obtain high tensile strength of 1000 MPa or more withoutconducting the off-line heat treatment, the content of Mn is preferablywithin the range of 2.5 to 4.0%, and more preferably within the range of2.5 to 3.5%.

[0029] Al: 0.005% to 0.10%

[0030] Al is an element which effects deoxidization and also makesgrains fine. Due to this grain-refining effect, Al makes the structurefine at the stage of mother tube, thereby further enhancing the effectof the present invention. In order to reliably achieve theaforementioned effect, the content of Al must be no less than 0.005%.However, when the content of Al exceeds 0.10%, the amount of oxide-basedinclusion is increased and cleanness of the steel deteriorates.Accordingly, in the present invention, the content of Al is restrictedwithin the range of 0.001 to 0.10%. The content of Al is preferably inthe range of 0.015 to 0.06%.

[0031] In addition to the aforementioned base composition, it ispreferable that at least one alloy element group selected from the groupconsisting of Composition A, Composition B and Composition C describedbelow is contained, according to necessity.

[0032] Composition A: at least one type of element selected from thegroup consisting of: no more than 1% of Cu; no more than 1% of Ni; nomore than 2% of Cr; and no more than 1% of Mo.

[0033] Cu, Ni, Cr and Mo are elements which increase strength of thesteel. These elements may be contained solely or as a combination of twoor more types, according to necessity. These elements serve for loweringthe transformation temperature and making the structure fine. However,when the content of Cu is too much (specifically, more than 1%), the hotworkability of the steel deteriorates. Ni increases tensile strength andimproves toughness. However, when the content of Ni exceeds 1%, theeffect achieved by Ni reaches the plateau and hardly improves any morehowever the content of Ni is increased. When the content of Cr or thatof Mo is too much (specifically, when the content of Cr exceeds 2% orwhen the content of Mo exceeds 1%), not only the weldability andductility of the steel deteriorate, but also the production cost of thesteel increases. Accordingly, it is preferable that the Cu content is nomore than 1%, the Ni content is no more than 1%, the Cr content is nomore than 2%, and the Mo content is no more than 1%. It is morepreferable that the Cu content is in the range of 0.1 to 0.6%, the Nicontent is in the range of 0.1 to 0.7%, the Cr content is in the rangeof 0.1 to 1.5%, and the Mo content is in the range of 0.05 to 0.5%.

[0034] Composition B: at least one type of element selected from thegroup consisting of: no more than 0.1% of Nb; no more than 0.5% of V; nomore than 0.2% of Ti; and no more than 0.003% of B.

[0035] Nb, V, Ti and B are elements which are precipitated as carbides,nitrides or carbo-nitrides thereby contributing to strengthing of thesteel. In particular, in a steel tube having a welded portion which isheated to a high temperature, the precipitates of these elements makegrains fine during the heating process at the time of welding, serve asprecipitation nuclei of ferrite during the cooling process of welding,and effectively prevent the welded portion from becoming hard. Theseelements may be added solely or as a combination of two or moreelements, according to necessity. However, when these elements are addedtoo much, the weldability and ductility of the steel are bothdeteriorated. Accordingly, in the present invention, it is preferablethat the content of Nb is restricted to no more than 0.1%, the contentof V is restricted to no more than 0.5%, the content of Ti is restrictedto no more than 0.2%, and the content of B is restricted to no more than0.003%. More preferably, the content of Nb is in the range of 0.005 to0.05%, the content of V is in the range of 0.05 to 0.1%, the content ofTi is in the range of 0.005 to 0. 10%, and the content of B is in therange of 0.0005 to 0.002%.

[0036] Composition C: at least one selected from the group consistingof: no more than 0.02 mass % of REM; and no more than 0.01 mass % of Ca.

[0037] REM and Ca are crystallized as sulfides, oxides or oxi-sulfides,make the shape of the inclusion spherical thereby improving theformability, and effectively prevent the welded portion of a steel tubefrom becoming hard. REM, Ca may be added solely or as a combination oftwo elements, according to necessity in the present invention. However,when the content of REM exceeds 0.02% or the content of Ca exceeds0.01%, there will be present too much inclusion in the steel, wherebythe cleanness and ductility of the steel are deteriorated. Accordingly,it is preferable that the content of REM is restricted to no more than0.02% and the content of Ca is restricted to no more than 0.01%. Whenthe content of REM is less than 0.004% or when the content of Ca is lessthan 0.001%, the aforementioned effects by REM, Ca may not besufficient. Therefore, it is preferable that the content of REM is noless than 0.004% and the content of Ca is no less than 0.001%.

[0038] The remainder other than the aforementioned elements of thecomposition is constituted of Fe and unavoidable impurities. Examples ofthe unavoidable impurities include: no more than 0.025% of P; no morethan 0.020% of S; no more than 0.010% of N; and no more than 0.006% ofO.

[0039] P: 0.025% or Less

[0040] It is preferable that the content of P is reduced as much aspossible because P is locally segregated in grain boundary anddeteriorates ductility of the steel. However, the presence of P isacceptable if the content of P is no more than 0.025%.

[0041] S: 0.020% or Less

[0042] It is preferable that the content of S is reduced as much aspossible because S increases the amount of sulfides and deterioratescleanness of the steel. However, the presence of S is acceptable if thecontent of S is no more than 0.020%.

[0043] N: 0.010% or Less

[0044] It is preferable that the content of N is reduced as much aspossible because N deteriorates weldability property of the steel.However, the presence of N is acceptable if the content of N is no morethan 0.010%.

[0045] O: 0.006% or Less

[0046] It is preferable that the content of O is reduced as much aspossible because O deteriorates cleanness of the steel. However, thepresence of O is acceptable if the content of O is no more than 0.006%.

[0047] The steel tube of the present invention has a structure which isconstituted of martensite and/or bainite or a structure which is amixture of martensite and/or bainite and ferrite. The martensite and/orbainite of the aforementioned structure is a transformation productobtained as a result of transformation of the deformed austenite (γ)which has been diameter-reducing-rolled, and significantly contributesto achieving higher strength and lower yield ratio (YR) and improvingthe three-point-bending property. In the present invention, thestructure may include ferrite in addition to the primary phase ofmartensite and/or bainite. It is preferable that the content of ferrite,expressed as the area ratio, is no more than 20%. When the amount offerrite exceeds 20% by the area ratio, the high strength of the desiredlevel cannot be reliably obtained. Accordingly, the amount of ferrite ispreferably no larger than 20% by the area ratio.

[0048] Next, the method of producing the steel tube of the presentinvention will be described hereinafter.

[0049] Although the method of producing the steel tube of the presentinvention employs a steel tube having “a specific composition” as amother steel tube, the method of producing the mother steel tube(tubeforming) is not particularly restricted. Examples of the method ofproducing the mother steel tube include: the electric resistance weldingwhich utilizes the high frequency current in cold roll forming or hotroll forming (the mother tube of such a type is called “electricresistance welded tube”, and especially called “hot electric resistancewelded tube” in the case of hot rolling); the solid phase pressurewelding in which both edge portions of an open tube are heated to thesolid phase pressure welding temperature range, whereby the edgeportions are pressure-welded (the mother tube of such a type is called“solid phase pressure welded tube); the butt-welding (the mother tube ofsuch a type is called “butt-welded tube”); and the Mannesmann typepiercing process (the mother tube of such a type is called “seamlesssteel tube”). Any of the aforementioned methods can be suitably used.

[0050] The mother steel tube having the aforementioned composition issubjected to a diameter-reducing rolling process in which the totaldiameter-reduction rate is no less than 20% and the temperature at whichthe diameter-reducing rolling process is finished is no higher than 800°C., preferably after being subjected to the heating or soakingtreatment. The temperature at which the heating or soaking treatment iscarried out is not particularly restricted, as long as the temperatureat which the diameter-reducing rolling process is finished is no higherthan 800° C. In the case in which the mother steel tube is once cooledto the room temperature, the heating treatment must be carried out.However, in this case, the temperature at which the heating treatment isconducted may be flexibly adjusted so that the temperature at which thediameter-reducing rolling process is finished is no higher than 800° C.,preferably within the “α+γ” two-phase range. For example, thetemperature at which the heating treatment is conducted may beadjustingly selected between Ac₃ transformation point and Ac₁transformation point or at Ac₃ transformation point or higher, and thencooled, so that the temperature at which the diameter-reducing rollingprocess is finished is no higher than 800° C., preferably within the“α+γ” two-phase range. In a case in which the mother steel tube isproduced in the hot roll forming or warm roll forming, the mother steeltube may be directly subjected to re-heating or soaking treatment beforethe mother steel tube is cooled to the room temperature, so that thetemperature at which the diameter-reducing rolling process is finishedis no higher than 800° C., preferably within the “α+γ” two-phase range.

[0051] When the total diameter-reduction rate is less than 20%, thedeformation of the austenite is insufficient and the low-temperaturetransformation phase (martensite or bainite) produced thereafter doesnot have sufficient strength, whereby tensile strength of the steelcannot be raised to 1000 MPa or higher.

[0052] The temperature at which the diameter-reducing rolling is carriedout is set so that the temperature at which the diameter-reducingrolling process is finished is no higher than 800° C. The temperature atwhich the diameter-reducing rolling is carried out is preferably setwithin the “α+γ” two-phase range.

[0053] When the temperature at which the diameter-reducing rollingprocess is finished exceeds 800° C., the rolling strain provided to theaustenite is instantly lost, whereby the low-temperature transformationphase (martensite or bainite) produced as a result of transformationfrom the austenite does not have sufficient strength and thus the hightensile strength TS of 1000 MPa or more cannot be achieved. In order toachieve such a high strength, the temperature at which thediameter-reducing rolling process is finished is preferably no lowerthan the temperature at which the martensite or bainite transformationis completed.

[0054] After being reduced, the mother steel tube is cooled according tothe conventional, standard method. This cooling process may be performedby way of either air or water.

[0055] In the present invention, the diameter-reducing rolling ispreferably rolling under lubrication (lubrication rolling). Byconducting lubrication rolling as the diameter-reducing rolling, thedistribution of strain in the thickness direction is made uniform, thestructure can be made uniformly fine in the thickness direction, and theformation of the texture can also be made uniform in the thicknessdirection. On the contrary, in the case of non-lubrication rolling, therolling strain concentrates at the material surface layer portion due tothe shearing effect, whereby the structure is formed non-uniformly inthe thickness direction.

[0056] The method of diameter-reducing-rolling is not particularlyrestricted. In the present invention, rolling by a tandem kaliberrolling mills (which are generally called “Reducer”) is preferable.

EXAMPLES

[0057] A hot rolled steel sheet (1.8 or 2.3 mm thickness) having thecomposition shown in Table 1 was electric resistance welded, whereby awelded steel tube (a electric resistance welded tube having outerdiameter of 58 mmφ) was produced. The obtained welded steel tube wasused as mother steel tube. The mother steel tube was subjected to theheating treatment, then to the diameter-reducing rolling process underthe conditions shown in FIG. 2, whereby a product tube was obtained. Thediameter-reducing rolling was carried out by using a reducer in whichrolling mills were tandem-arranged.

[0058] The structure, the tensile properties and the three-point-bendingproperty of the obtained product tubes were examined.

[0059] (1) Structure

[0060] A test piece was taken from each product tube. The structure ofthe test piece was photographed, at a section of the test pieceperpendicular to the longitudinal direction of the tube, by using anoptical microscope and a scanning electron microscope. For each of themicrograph structure thus obtained, the types of the constituentstructures and the percentage of respective constituent structures wereobtained by using an image analyzing device.

[0061] (2) Tensile Properties

[0062] A JIS No. 11 test piece (a tube-shaped test piece, the gaugelength being 50 mm) was taken from each product tube, in thelongitudinal direction of the product tube. A tensile test was carriedout according to the regulation of JIS Z 2241, whereby yield strengthYS, tensile strength TS and elongation El were obtained.

[0063] (3) Three-Point-Bending Property

[0064] A (tube-shaped) test piece was taken from each product tube. Foreach test piece, a three point bending test was carried out, as shown inFIG. 1, with the span L being 800 mm or 980 mm and the curvature radiusR of the pressing tool being 152.4 mm, whereby the relationship betweenthe load and the pressing amount, as well as the buckling limit pressingamount 6 max, which was the maximum pressing amount before bucklingoccurred, was obtained. In addition, by using the pressing load-pressingamount curve thus obtained, the area between “the pressing load-pressingamount curve from the start of pressing to the buckling limit pressingamount” and “the amount of deformation” axis was obtained, whereby theabsorption energy E was defined.

[0065] The obtained results are shown in Table 2.

[0066] All of the examples of the present invention exhibit excellentlyhigh tensile strength (1000 MPa or more), excellently highthree-point-bending buckling limit pressing amount, and excellently highthree-point-bending absorption energy. On the other hand, in thecomparative examples whose compositions are beyond the range of thepresent invention, the buckling limit pressing amount and the amount ofthe absorption energy are both low and the three-point-bending propertyis poor, as compared with the corresponding present examples of the samedimension. TABLE 1 Steel Chemical composition (mass %) No. C Si Mn P SAl Cu, Nl, Cr, Mo Nb, V, Ti, B REM, Ca A 0.14 0.18 2.99 0.018 0.005 0.03Cr: 0.10 Nb: 0.020, Ti: 0.015 — B 0.09 0.21 3.10 0.021 0.005 0.04 Cr:0.15 Nb: 0.039 — C 0.16 0.25 2.50 0.016 0.003 0.03 Cu: 0.12, Nl: 0.15,Mo: 0.15 NB: 0.015, V: 0.08 Ca:0.0010 D 0.22 0.19 2.00 0.018 0.003 0.03Cr: 0.2 Ti: 0.012, B: 0.0009 — E 0.22 0.35 2.80 0.018 0.003 0.03 — — — F0.25 0.35 1.50 0.018 0.003 0.03 Cr: 0.5, Mo: 0.10 Nb: 0.022 —

[0067] TABLE 2 mother steel tube Conditions of diameter-reducing rollingHeating/ Temperature at Temperature at Steel Outer Tube Soaking whichrolling Diameter- which rolling Product tube tube Steel diameterThickness temperature was started reducing rate was finished Coolingafter Outer diameter Thickness No No (mm) (mm) (° C.) (° C.) (%) (° C.)rolling (mm) (mm) 1 A 28.6 1 6 — — — as ERW — 28 6 1 6 2 58 0 1 8 800730 51 680 Water 28 6 1 6 cooling 3 800 740 51 700 Water 28 6 1 6cooling 4 850 780 51 730 Water 28 6 1 6 cooling 5 31 8 1 6 — — — as ERW— 31.8 1 6 6 58 0 1 8 750 700 45 650 Left to be 31 8 1 6 cooled 7 750700 45 650 Left to be 31 8 1 7 cooled 8 58 0 2 3 750 700 45 650 Left tobe 31 8 2 0 cooled 9 31 8 2 0 — — — as ERW — 31 8 2 0 10 31.8 1 6 — — —as ERW — 31 8 1 6 11 750 710 45 650 Left to be 31 8 1.6 cooled 12 58 01 8 860 820 45 750 Left to be 31.8 1 6 cooled 13 980 930 45 850 Left tobe 31 8 1 6 cooled 14 B 31.8 1 6 — —  0 as ERW — 31 8 1.6 15 58 0 1 8870 830 45 750 Left to be 31 8 1 6 cooled 16 1050 980 45 900 Left to be31 8 1 6 cooled 17 C 58 0 2.3 800 750 45 700 Left to be 31 8 2 0 cooled18 D 58 0 2 3 800 750 45 700 Left to be 31 8 2 0 cooled 19 E 58 0 2 3800 750 45 700 Left to 31 8 2 0 be cooled 20 F 58 0 2 3 800 750 45 700Left to be 31 8 2 0 cooled Structure Three-point-bending property SteelTensile properties Ferrite Bending Buckling limit Absorbed tube Steel YSTS YR EL area rate span L pressing energy before No No MPa MPa % % Type(%) (mm) amount δ (mm) buckling E (J) Note 1 A 1093 1190 92  7 B* — 800 80 350 Comparative Example 2 739 1337 55 22 M + F 8 800 125 450 PresentExample 3 882 1370 64 18 M, B — 800 100 460 Present Example 4 660 120155 24 M + F 12  800 130 420 Present Example 5 1129 1213 93  9 B* — 800 60 385 Comparative Example 6 844 1291 65 18 M, B + F 6 800  76 465Present Example 7 853 1305 65 18 M, B + F 8 800  91 724 Present Example8 980 1390 71 16 M, B + F 9 800 100 960 Present Example 9 1145 1220 9410 B* — 800  67 649 Comparative Example 10 1129 1213 93  9 B* — 980  76398 Comparative Example 11 1066 1396 76 19 M, B + F 6 980 100 561Present Example 12 830 1089 76 18 M, B + F 6 980 110 470 Present Example13 602 990 61 15 B* — 980  95 395 Comparative Example 14 B 921 1090 8414 B* — 980  85 376 Comparative Example 15 666 1009 66 22 M, B + F 7 980100 480 Present Example 16 600 890 67 24 B* — 980  95 365 ComparativeExample 17 C 1076 1380 78 18 M + F 3 800 105 1160  Present Example 18 D1013 1350 75 19 M + F 3 800 115 1200 Present Example 19 E 1078 1400 7716 M, B + F 10 800 110 1250 Present Example 20 F 679 970 70 16 M, B + F25 800 70 700 Comparative Example

INDUSTRIAL APPLICABILITY OF THE PRESENT INVENTION

[0068] According to the invention, the production efficiency can beenhanced and the production cost can be reduced in the steel tubeproduction, without necessitating any off-line heat treatment. Inaddition, according to the present invention, the three-point-bendingabsorbed energy is increased and thus the thickness of the steel tubecan be made thinner and the weight of a automobile can be significantlyreduced, which is extremely advantageous in industrial terms.

1. A steel tube for reinforcing a automobile door, having a compositioncomprising: 0.05 to 0.30 mass % of C; 0.01 to 2.0 mass % of Si; 1.8 to4.0 mass % of Mn; 0.005 to 0.10 mass % of Al; and the remainder as Feand unavoidable impurities, wherein the steel tube has tensile strengthof no less than 1000 MPa and is excellent in three-point-bendingproperty.
 2. A steel tube for reinforcing a automobile door according toclaim 1, wherein the steel tube has a structure which is constituted ofmartensite and/or bainite, and the martensite and/or bainite is atransformation product obtained as a result of transformation of adeformed austenite.
 3. A steel tube for reinforcing a automobile dooraccording to claim 1, wherein the steel tube has a structure which is amixture of martensite and/or bainite and ferrite, and the martensiteand/or bainite is a transformation product obtained as a result oftransformation of a deformed austenite.
 4. A steel tube for reinforcinga automobile door according to claim 3, wherein the content of ferrite,expressed as the area ratio, is no more than 20%.
 5. A steel tube forreinforcing a automobile door according to claim 1 to 4, wherein theyield ratio of the steel tube is no larger than 80%.
 6. A steel tube forreinforcing a automobile door of any according to claims 1 to 4, whereinthe steel tube has at least one composition selected from the groupconsisting of composition A, composition B and composition C describedbelow, in addition to the aforementioned composition. Composition A: atleast one type of element selected from the group consisting of: no morethan 1 mass % of Cu; no more than 1 mass % of Ni; no more than 2 mass %of Cr; and no more than 1 mass % of Mo. Composition B: at least one typeof element selected from the group consisting of: no more than 0.1 mass% of Nb; no more than 0.5 mass % of V; no more than 0.2 mass % of Ti;and no more than 0.003 mass % of B. Composition C: at least one selectedfrom the group consisting of: no more than 0.02 mass % of REM; and nomore than 0.01 mass % of Ca.
 7. A method of producing a steel tube forreinforcing a automobile door, comprising the steps of: preparing amother steel tube having a composition which includes: 0.05 to 0.30 mass% of C; 0.01 to 2.0 mass % of Si; 1.8 to 4.0 mass % of Mn; 0.005 to 0.10mass % of Al; and the remainder as Fe and unavoidable impurities;subjecting the mother steel tube to a heating or soaking treatment; andthereafter, subjecting the mother steel tube to a diameter-reducingrolling process in which the total diameter-reduction rate is no lessthan 20% and the temperature at which the diameter-reducing rollingprocess is finished is no higher than 800° C.
 8. A method of producing asteel tube for reinforcing a automobile door according to claim 7,wherein the steel tube has at least one composition selected from thegroup consisting of composition A, composition B and composition Cdescribed below, in addition to the aforementioned composition.Composition A: at least one type of element selected from the groupconsisting of: no more than 1 mass % of Cu; no more than 1 mass % of Ni;no more than 2 mass % of Cr; and no more than 1 mass % of Mo.Composition B: at least one type of element selected from the groupconsisting of: no more than 0.1 mass % of Nb; 0.5 mass % of V; no morethan 0.2 mass % of Ti; and no more than 0.003 mass % of B. CompositionC: at least one selected from the group consisting of: no more than 0.02mass % of REM; and 0.01 mass % of Ca.